Plug your rca's from your 4 channel into your bass amp with your subs hooked up to the big amp. That will tell you if you have a set of bad rca's on the deck itself..
Hey bush when your waiting for the old trans fluid to flush out.Now you've added the proper amount of new fluid to keep it full while its running and flushing can you distinctly tell when the old stuff is all out? Since your adding new trans to the old doesn't it mix together? Can you still see...
TRy hooking one of your subs to your 4 channel..See if you get sound from that amp. Then you can narrow it down to wiring on the subs/bad rca or ground to the amp itself..